A sample of radio-loud AGN in the Sloan Digital Sky Survey

A sample of 2712 radio–luminous galaxies is defined from the second data release of the Sloan Digital Sky Survey (SDSS) by cross–comparing the main spectroscopic galaxy sample with two radio surveys: the NRAO VLA Sky Survey (NVSS) and the Faint Images of the Radio Sky at Twenty centimetres (FIRST) survey. The comparison is carried out in a multi– stage process and makes optimal use of both radio surveys by exploiting the sensitivity of the NVSS to extended and multi–component radio sources in addition to the high angular resolution of the FIRST images. A radio source sample with 95% completeness and 98.9% reliability is achieved, far better than would be possible for this sample if only one of the surveys was used. The radio source sample is then divided into two classes: radio–loud AGN and galaxies in which the radio emission is dominated by star formation. The division is based on the location of a galaxy in the plane of 4000Å break strength versus radio luminosity per unit stellar mass and provides a sample of 2215 radio–loud AGN and 497 star forming galaxies brighter than 5 mJy at 1.4 GHz. A full catalogue of positions and radio properties is provided for these sources. The local radio luminosity function is then derived both for radio–loud AGN and for star-forming galaxies and is found to be in agreement with previous studies. By using the radio to far-IR correlation, the radio luminosity function of star forming galaxies is also compared to the luminosity function derived in the far–infrared. It is found to agree well at high luminosities but less so at lower luminosities, confirming that the linearity of the radio to far-IR correlation breaks down below about 10W Hz at 1.4 GHz.